[gn] Make FrontendTests depend on AllTargetsDescs
The CMake build merged most clang unittests into a single binary.
The gn build doesn't (yet?) do this, but as part of that change,
FrontendTests somehow picked up a dependency on AllTargetsDescs,
see https://github.com/llvm/llvm-project/pull/134196#discussion_r2068617059
